A variety of alarm devices have heretofore been proposed for use in vehicles to indicate when the vehicle has been entered by unauthorized persons and/or when selected components of the vehicle are being tampered with, a representative sampling of such circuits being generally described at pages 82-83 of "Guidebook of Electronic Circuits" (1st Ed., 1974) and in an article appearing at page 32 of "Popular Electronics" magazine, January 1973 issue.
While these prior art alarm circuits are generally operable to sound an alarm, usually the horn of vehicle, when predetermined electrical circuits of the vehicle are energized (i.e. the ignition switch circuit), they have certain drawbacks. Many of these prior art circuits ae designed to be connected with, and operated by, only one particular element or circuit of the vehicle, whereby tampering with other components or parts of the vehicle would go undetected. For example, an alarm circuit such as that entitled Door-Switch Alarm at page 83 of the aforementioned "Guidebook of Electronic Circuits" is only activated when one of the dome light door switches of a vehicle is closed, but a thief may nevertheless enter the vehicle through a back door thereof which normally does not have a dome light door switch, or the thief may tamper with other parts of the vehicle without entering the vehicle (i.e., the gas tank), all without detection.
To avoid the drawbacks of single alarm triggering components, other known alarm circuits have been devised which are responsive to the energization of any one of a plurality of existing electrical components within the vehicle, such as the trunk light, the ignition switch and the headlamps. Such multitriggered alarm circuits are nevertheless always operable in response to exactly the same condition occurring in one or more of the different electrical components to which the circuit is connected. For example, such a circuit may be triggered when the 12-volt vehicle voltage is applied to any one of the selected triggering components, as would be the case when such components are activated by the existing switch therefor in the vehicle. However, as a practical matter, the operation and accessibility of the existing circuits of different components in vehicles makes it difficult if not impossible to conveniently connect an auxiliary alarm circuit to all such existing circuits. For example, it may be convenient to connect the alarm circuit to the existing ignition switch of the vehicle at a point where the alarm will be triggered when the ignition switch is closed and 12 volts is applied to the alarm circuit, whereas it may be convenient to connect the alarm circuit to the existing dome light door switch at a point where the alarm will be triggered when the door switch is closed and the alarm circuit is connected to ground. Thus, known alarm circuits which are responsive to only one trigger circuit condition that must be present upon the actuation of any one of a plurality of different components are not compatible, as a practical matter, with existing vehicle wiring circuits.
Finally, the rapidly rising cost of gasoline has resulted in a corresponding rise in incidents of gasoline thefts from the gasoline tanks of vehicles, even where attempts have been made to secure such gasoline tanks by locked gasoline fill tank caps and the like. Electrical alarm circuits for gas tanks, while obviously being desirable, have heretofore encountered two significant difficulties. First, there are generally no existing electrical components located at the gas tank fill pipe which can act to trigger an alarm circuit, and, secondly, if a conventional electrical vehicle switch is mounted at the gas tank fill pipe, for triggering an alarm circuit, there is an ever present danger that the volatile gasoline fumes inherently associated with the gas tank will be accidentally ignited by the electric circuit of such conventional switch.